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Shedding light on amorphous silicon

Scientists in Europe took a very close look at the thin amorphous silicon layers used in heterojunction and tandem solar cells, building a full picture of the material’s structure at the nanoscale. Their findings could help scientists solve the long-standing mystery of light induced degradation.

Scientists led by Germany’s Helmholtz Zentrum Berlin (HZB) examined the structure of hydrogenated amorphous silicon (a-Si:H) at a resolution of 0.8 nanometers. Using multiple microscopy techniques, the group was able to identify ‘order in the disorder’ of the material’s nanostructure, with ‘voids’ of missing atoms and areas of highly ordered particles, with hardly any hydrogen.

“With this study, we show that a-Si:H is by now means a homogenously amorphous material,” says HZB Professor Klaus Lips. “After over 50 years of intensive research on this topic, we know today that the a-Si∶H network is a complex mixture of amorphous and nanosized crystalline-like domains in which nanovoids and vacancies of different sizes are embedded.”

The research is described in the paper Quantification of Nanoscale Density Fluctuations in Hydrogenated Amorphous Silicon, published in Physical Review Letters. The group first identified ‘voids’ created by a few more than 10 missing atoms in the structure: “These voids arrange themselves into clusters with a recurrent distance of about 1.6 nanometers to each other,” explains the paper’s lead author Eike Gericke. And the concentration of such voids was shown to increase in a-Si:H layers deposited at faster rates.

The material is also shown to contain areas of highly ordered silicon (densely ordered domains, or DODs), with hardly any hydrogen present. And these areas were shown to be present in all of the a-Si:H samples analyzed, regardless of a specific structure or deposition technique.

Light-induced degradation

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The discovery validates earlier theories about the structure of a-Si:H, and could also help scientists to understand light-induced degradation mechanisms in silicon heterojunction cells. Hydrogen has long been speculated to play a key role in these, and the HZB researchers note that density fluctuations in a-Si:H can cause leakage current and lost performance in HJT cells.

HZB suggests that further work and possibly even higher resolution imaging of a-Si:H could be needed to fully understand the LID problem, but that its structural model offers a new way to understand the material. “We strongly suggest that the structural model presented here is directly linked to the light-induced formation of defect and this view of on the nanostructure has to be considered to describe the complex dynamics…” the paper concludes.

And improved understanding and control of the a-Si:H nanostructure could lead to better solar cells, as well as other applications in photonics and silicon batteries. “The DOD regions are able to reduce mechanical stress in the material and thus contribute to the stability of the a-Si:H thin film,” says Lips. “The voids on the other hand can promote electronic degradation of the semiconductor layers.”

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Source: pv magazine